LTNE条件下界面对流传热系数对部分填充多孔介质通道传热特性的影响

在多孔介质区考虑局部非热平衡,采用Brinkman-extended Darcy模型结合应力跳跃条件对部分填充多孔介质通道内流体传热特性进行分析。获得了各区域温度分布及Nusselt数解析解,并分析了各参数对温度及Nusselt数的影响。结果表明：界面对流传热系数H_s较小时,界面应力跳跃系数β和Darcy数Da的增加会减小流固两相间温差。而在高H_s下,Da减小也会减小两相温差。在Da、H_s和固流两相热导率之比K较大且空心率S（自由流体区高度与通道高度之比）和Biot数Bi较小时,流固两相间会在接近多孔介质区中部出现最大温差,而该最大温差会随着S增加和Da及H_s的减小向界面区移动。对于不同K与Bi,Nusselt数Nu与S的关系曲线存在不同的类型,与模型A（界面处多孔介质固相和流相根据各自温度梯度和热导率划分总热流）不同的是,采用模型C（界面处固相热流分配与自由流体区流相的热交换相关）所获得的Nu曲线类型与H_s有关。在K较小时,β对Nu的影响大于H_s对Nu的影响;而在K较大时,H_s对Nu的影响要远大于β对Nu的影响,且H_s增加会明显提高通道内的Nu。     

表面辐射对部分填充吸湿性多孔介质的封闭腔体内热湿耦合传递的影响

根据多孔介质热质传递原理, 基于有限元的方法数值分析了具有表面热辐射的部分填充吸湿性多孔介质的封闭腔体内部自然对流流动及热湿耦合传递过程, 探讨了表面发射率、Rayleigh数和Darcy数等参数对封闭腔体内部自然对流流动及热湿耦合传递过程的影响, 研究结果表明, 壁面热辐射的作用可以提高多孔介质内部的温度, 而且随着表面发射率的增大, 多孔介质内部的水分逐步向其右上角迁移和聚集。另外, Darcy数、多孔介质与空气的热导率比对方腔内部多孔介质的热量传递和水分迁移影响较小。     

多孔壁面微通道换热性能的实验研究

微通道散热器作为一种高效散热器件,广泛应用于微电子、光电、汽车、航天国防、能源等领域。针对传统光滑微通道传热面积小、换热性能偏低、沸腾迟滞等问题,本文提出一种多孔壁面微通道结构,并采用激光直写方法实现微通道多孔壁面的高效、稳定生成。该多孔壁面微通道显著增大了换热面积、促进流体的扰动、提供大量稳定沸腾核心,从而强化单相与两相沸腾传热。通过搭建微通道换热性能测试系统,测试对比了多孔壁面微通道与光滑微通道的单相对流、两相沸腾传热性能。发现多孔壁面微通道的Nu数相对于光滑微通道提升了21%~31%。在两相沸腾换热过程中,其粗糙多孔结构促进了沸腾气泡成核,其核态沸腾起始温度相比于光滑微通道降低了35%。同时粗糙多孔结构可以保证沸腾过程中的液体持续供给,从而大幅提升了沸腾换热能力,避免了干涸现象的提前发生,其两相沸腾换热系数相对于未处理的光滑微通道最大提升了83%。此外,还开展了不同流量下多孔壁面微通道的沸腾传热性能测试,发现在质量流率为G=500kg/(m²·s)下的沸腾换热系数相对于G=200kg/(m²·s)情况下最大提升了30%。     

NATURAL CONVECTION HEAT TRANSFER IN A POROUS LAYER WITH MULTIPLE PARTITIONS

Laminar natural convection heat transfer in inclined porous layers with multiple diathermal partitions is studied analytically and numerically. On the basis of the Darcy-Oberbeck-Boussinesq equations, the problem is solved analytically in the limit of a thin layer with constant flux boundary conditions. The relationship between overall heat transfer and the number of partitions N, their relative positions Nu and the angle of inclination φ of the system is determined

The critical Darcy-Rayleigh number R_c for the onset of convection in a bottom-heated horizontal system is predicted. The results are compared with limiting cases of the problem and are found to be in agreement. The influence of a thermal barrier which is sandwiched between two porous layers is also discussed. A numerical study of the same phenomenon, obtained by solving the complete system of governing equations, is conducted. A good agreement is found between the analytical predictions and the numerical simulation.     

REV尺度多孔介质格子Boltzmann方法的数学模型及应用的研究进展

综述了多孔介质表征体元尺度(REV)格子Boltzmann模型的研究进展,根据对多孔介质处理方式主要分为部分反弹模型和阻力模型两类,分析归纳了各类模型的优缺点。由于阻力模型中渗流的广义格子Boltzmann方程(GLBE)的作用力是基于GUO等的作用力模型,可以准确得到宏观方程,不存在离散误差,且模型的平衡分布函数和作用力项中都包含反应介质特性的孔隙率,因而应用最为广泛。本文还重点介绍了REV尺度多孔介质LBE模型在流动、传热、传质、化学反应及相变等过程中的具体应用,认为REV尺度多孔介质内的三传一反数学模型中需要加入孔隙尺度因素,在更大工程尺度上应该考虑过程参数的各向异性,展望了REV尺度多孔介质LBE模型的发展和应用前景。     

多孔介质/纯流体耦合区域内可压缩气体的流动

采用统一形式的修正N-S方程描述多孔介质/纯流体耦合区域的流动,提出了方程中用于处理多孔介质内流动的源项确定方法,并利用成熟的CFD技术对圆管内具有前后台阶的耦合区域内可压缩气体的流动进行了数值模拟,得到了与实验吻合的计算结果.     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 2

Nonuniformities in porous materials can play a significant role in the convective and diffusive transport of fluid, heat, and mass. This study provides experimental results and corresponding numerical simulation results. The experiments report continuous data in transient test runs with measurements of temperature and velocity at distributed locations in the domain immediately downstream of the porous materials. An increasing degree of nonuniformity was found to produce a lower drying rate as well as an earlier onset of falling rate drying. The numerical model provides results of the effects of different nonhomogeneities such as distributed holes or distributed regions of varying permeability and moisture content. Comparisons between numerical and experimental test results indicate general agreement with differences with regard to details of the drying curve features. This provides a tool for studying the role of nonuniformity in fluid flow and heat and mass transfer in porous media.     

冰点以下多孔介质中气体水合物的生成动力学研究进展

青藏高原冻土区储存着大量的天然气水合物资源,CO₂置换开采冻土区的天然气水合物可实现天然气水合物的安全开采和温室气体CO₂的地层封存。冰点以下多孔介质中气体水合物的生成动力学,是冻土区天然气水合物置换开采研究领域的难点和热点问题。本文全面综述了冰点以下多孔介质中气体水合物的生成动力学研究进展,讨论了不同体系冰点以下多孔介质中气体水合物的形成机理及其生成特性;详述了冰生成水合物机理及其冰粉/多孔介质体系中气体水合物的生成特性,分析了冰点以下多孔介质中气体水合物生成动力学研究尚待完善和改进的地方。最后本文指出冰点以下多孔介质中水合物的生成过程是由传热、传质等多种因素所控制,揭示不同过程的主导因素及其影响规律是今后研究的重点方向。目前对冰点以下多孔介质中水合物的生成特性及机理的认识尚未成熟,仍需深入研究。     

HEAT AND MASS TRANSFER IN STAGNATION-POINT FLOW OF A POLAR FLUID TOWARDS A STRETCHING SURFACE IN POROUS MEDIA IN THE PRESENCE OF SORET,DUFOUR AND CHEMICAL REACTION EFFECTS

This work is focused on the numerical solution of steady boundary-layer stagnation-point flow of a polar fluid towards a stretching surface embedded in porous media in the presence of the effects of Soret and Dufour numbers and first-order homogeneous chemical reaction. The governing boundary-layer equations of the problem are formulated and transformed into a self-similar form. The obtained equations are solved numerically by an efficient, iterative, tri-diagonal, implicit finite-difference method. Both assisting and opposing flow conditions are considered. Comparisons of the present numerical results with previously published work under limiting cases are performed and found to be in excellent agreement. Representative results for the fluid velocity, angular velocity, temperature, and solute concentration profiles as well as the local heat and mass transfer rates for various values of the physical parameters are displayed in both graphical and tabular forms.     

A numerical study on heat transfer enhancement and design of a heat exchanger with porous media in continuous hydrothermal flow synthesis system

The aim of this study is to use a new configuration of porous media in a heat exchanger in continuous hydrothermal flow synthesis (CHFS) system to enhance the heat transfer and minimize the required length of the heat exchanger.For this purpose,numerous numerical simulations are performed to investigate performance of the system with porons media.First,the numerical simulation for the heat exchanger in CHFS system is validated by experimental data.Then,porous media is added to the system and six different thicknesses for the porous media are examined to obtain the optimum thickness,based on the minimum required length of the heat exchanger.Finally,by changing the flow rate and inlet temperature of the product as well as the cooling water flow rate,the minimum required length of the heat exchanger with porous media for various inlet conditions is assessed.The investigations indicate that using porous media with the proper thickness in the heat exchanger increases the cooling rate of the product by almost 40％and reduces the required length of the heat exchanger by approximately 35％.The results also illustrate that the most proper thickness of the porous media is approximately equal to 90％ of the product tube's thickness.Results of this study lead to design a porous heat exchanger in CHFS system for various inlet conditions.     

The Role of Nonuniformity in Convective Heat and Mass Transfer through Porous Media,Part 1

Through-air drying is commonly used in the drying of high-quality tissue and towel products. A representative elementary volume method was used to model the fluid flow and heat and mass transfer during through drying in heterogeneous porous biobased materials such as tissue and towel products. Results of flow both upstream and downstream of a modeled porous sheet allowed visualization of the effects of mixing at the top and bottom of the porous medium. The effect of initial nonuniformity on fluid flow and convective heat and mass transfer in heterogeneous porous media was studied. The effect of material nonhomogeneity and associated transport properties on moisture content of the porous material as a function of drying time was studied. Modeling results indicate that for the first time it is possible to simulate the effect of nonuniformity on fluid flow and convective heat and mass transfer in porous media during through-air drying of paper. Moisture and structural nonuniformity contributing to nonuniformity in air flow might contribute significantly to drying nonuniformity. Depending on the moisture regimes and degree of saturation of the convective medium, heat and mass transfer coefficients may have varying effects on the overall drying.     

非热平衡多孔介质内反应与传热传质耦合过程

采用局部热不平衡假设,对发生强吸热化学反应的多孔介质体系建立了反应与传热、传质耦合问题的数学模型,采用Ergun-Forchheimer-Brinkman方程描述多孔介质中的流体流动.运用交替方向隐式(ADI)方法对模型离散求解,并采用文献中的实验数据对模型进行验证.计算了不同条件下颗粒物料层内气体和固体骨架的温度场、产物气体浓度场以及固体转化率分布,以得到多孔介质体系内固有化学反应时的传热、传质规律.结果表明,不能忽略固体骨架与流体间的温度差.入口渗流速度、入口气体温度以及固体颗粒尺寸是影响系统反应特性的重要参数.研究结果对具有强吸热反应的固定床反应器的设计和运行具有一定的参考作用.     

多孔介质对流干燥机理及其模型

在对现有的多孔介质对流干燥传热、传质模型归类分析的基础上,从介质内部热湿迁移机制出发,建立了能较完善、较准确地描述多孔介质在恒速段及降速段热质传递规律的“三耦合-六场量”混合理论模型.同时针对干燥问题数值模拟中的移动边界问题,提出了一种迭代修正的思想,并发展了相应的数值计算方法.对砖的干燥模拟计算结果表明,本文的模型较其他模型具有更好的精确性.     

LPG埋地储罐泄漏渗流扩散多相混合数值模型及模拟（Ⅰ）模型建立

在混合模型理论基础上,建立LPG（liquefied petroleum gas）在非饱和砂土中渗流扩散的传热、传质模型,研究LPG埋地储罐在砂土介质中发生泄漏时LPG渗流扩散的物理过程。该模型综合考虑了LPG泄漏相态、重力、黏滞力、毛细压力、扩散、非达西效应、气液相之间相互作用等影响因素的作用;在Forchheimer方程基础上,推导出多相混合流的非达西流动方程,并根据LPG实际渗流过程中流动状态的变化,将多相流非达西系数根据Reynolds数变化进行相应的改变,实现对多相流体从非达西流动向达西流动的转变现象的描述,能够模拟多种流动状态变化,扩展了模型的适用范围;根据LPG不溶于水的特性,简化了组分方程,推导出LPG体积浓度计算式。该模型能描述LPG埋地储罐泄漏后在砂土中流动趋势和对周围造成的影响,为埋地储罐场地选取、周边区域规划和事故预防、预测、应急提供依据。     

基于微量热法对多孔碳与双氧水相互作用过程的传质阻力分析

多孔材料作为催化剂对现代化学工业起到重要推动作用,但其纳米受限孔道造成的界面传递问题不容忽视。直接法合成双氧水（H₂O₂）过程中,揭示H₂O₂脱附过程传递与反应竞争博弈的介尺度机制是提高产率的关键。线性非平衡热力学为解耦界面扩散及反应提供了统一框架,但缺少合适通量测定方法。因此,本文设计多孔碳与H₂O₂相互作用的微量热实验,结合分子模拟及孔结构表征实验揭示多孔碳材料的界面传递结构,实现了非平衡热力学的定量传质阻力分析,进一步获取了表界面浓度场的动态变化。研究结果表明：微量热法是定量解耦并揭示扩散-反应机制的有效线性非平衡热力学阻力分析方法;介孔结构、生物骨架结构及担载1%（质量） Pd元素均能增强H₂O₂在多孔碳中的传质通量,但实现超高通量需要扩散与反应阻力的匹配;非平衡热力学阻力解耦方法是揭示多相反应过程介尺度机制的重要定量描述方法,有望为过程的调控及优化提供理论依据。     

Experimental and numerical study of applicability of porous combustors for HCl synthesis

Porous combustors have been studied intensively concerning the combustion of natural gas. The advantages of combustion in porous inert media, such as low emissions, high power turndown ratio of typically 10:1 and compactness, can also be used for different chemical gas phase reactions, e.g. the HCl synthesis from H₂ and Cl₂. The advantages of porous reactors result from the heat transport properties of the porous medium, i.e. emissivity and conductivity. Heat transport mechanisms and chemical reactions were implemented in a numerical code in order to investigate the H₂/Cl₂ system. Important parameters of the reaction, e.g. the laminar flame speed and the adiabatic flame temperature, are higher for the H₂/Cl₂ reaction compared to the CH₄/air combustion. By studying the influence of H₂O and HCl as inert components it was shown by numerical investigations that the maximum temperature could be decreased to a level, which makes the usage of a porous reactor feasible. A porous reactor for laboratory use was tested with O₂/CH₄/N₂ combustion, which delivers even higher adiabatic temperatures and flame speeds than the H₂/Cl₂ reaction. Finally, experiments with H₂/Cl₂/HCl reaction were carried out and first results are presented.     

纸张干燥中湿分扩散系数的估算

纸张干燥过程涉及到多孔介质的热质传递,如何确定质量扩散系数是所建立的多孔物料湿分扩散模型能否进行数值计算的关键。按Liukov公式将湿分扩散系数视为含湿质量分数的非线性函数,在恒温下进行纸张干燥实验,通过比较湿分蒸发质量的测量值与理论计算值,采用多变量寻优的方法对多孔介质一维情况下的湿分扩散系数进行估算,得到了实验条件下的纸张湿分扩散系数的计算公式。并进一步确定了纸张中的湿分扩散系数与含湿质量分数和温度之间的函数关系式。     

多孔介质内部传热传质规律的研究进展

多孔介质中的热质交换理论及其实验研究是一个涉及面广、研究难度大而又颇具工程应用价值的课题。本文对多孔介质内部传热传质规律的研究从理论和实验两方面作了简要的综述及展望,并对一些描述热质传递过程的数学模型及方程作了介绍。     

微通道内超临界氮气三维热流场实验与数值模拟

利用实验与数值模拟相结合的方法研究了超临界氮气（SCN₂）三维热流场特性,在用实验数据验证数值方法可靠性基础上,分析了压力（3.6~7 MPa）和质量流速［800~1200 kg/（m²?s）］对SCN₂对流传热特性的影响,揭示了微通道圆管不同圆周方向上SCN₂热流场规律。结果表明：在低压力和高质量流速下,同一轴向位置处,径向内壁温最大值出现在圆管90°处;质量流速越大,内壁温最大值和对流传热系数最小值由圆管180°向90°处发生了转移;当浮升力系数Gr*/Re²>1时,浮升力有利于强化圆管底部区域流体传热能力;基于获得的数据,提出了一个新的适合预测微通道圆管内SCN₂对流传热特性的无量纲换热关联式,预测误差小于20%。研究结果为微通道换热器优化设计提供了参考。     

Heat and mass transfer in granular potash fertilizer with a surface dissolution reaction

Potash is a widely used granular fertilizer and when exposed to high humidities it readily adsorbs water vapour forming a liquid electrolyte solution on each particle. Heat and mass transfer due to air flow through granular potash beds is studied experimentally and numerically. A one dimensional experimental setup is used to measure the temperature and air humidity response and mass gain of a potash bed subjected to a change in air flow. A porous media mathematical model is developed to predict the transient temperature and moisture content distributions. The processes are modelled as nonequilibrium heat and mass transfers between the porous solid and air flow gaseous phases. The state of the surface electrolyte solution is modelled by the thermodynamics of electrolyte solutions. Experimental and numerical results show non‐equilibrium internal moisture and heat transfer processes exist with significant differences in the pore air and particle temperature and surface relative humidity.